A dynamic range of a receiver, a ratio between the largest signal and the smallest signal which can be detected by the receiver, is a critical performance parameter of receivers. The receiver linearity is determined by the largest signal applied to the receiver; for any signal larger than the largest signal, one or more stages in the receiver chain enter a saturation region and thus introduce distortion. The receiver sensitivity is determined by the smallest signal applied to the receiver, which in turn is determined by a receiver noise figure. Large dynamic ranges are often required, allowing the receiver to operate over a variety of input signal conditions. Therefore, high receiver linearity is required together with a low noise figure.
Being the first active circuit after an antenna, a LNA is a crucial block for a receiver or transceiver. To improve receiver sensitivity or reduce the receiver noise figure, a LNA with a high power gain, which deemphasizes the noise distributions further downstream in the receiver, is required. However, a LNA with high power gain amplification is not only a target of in-band signals but also any out-of-band interference near the tone frequency of the target signal. If the target signal or the out-of-band interferers are strong, the LNA may render the LNA itself or following mixers saturated, inadvertently deteriorating the linearity of the receiver. It can be seen from the above analysis that, a LNA prefers a high power gain in consideration of receiver noise figure, but in contrast, favors low power gain in consideration of receiver linearity. A compromised design is to have a LNA with a moderate power gain to balance the linearity and the noise figure. A better solution, though, is to have a LNA whose power gain can be switched or varied depending on the strength of the in-band signal or the out-of-band interferers.
FIG. 1 demonstrates a variable-gain LNA according to the prior art. LNA 100 is a differential amplifier with a common-source amplifier configuration. A control voltage VCTRL controls transistors MSL and MSR to steer currents that MOS transistors MAL and MAR conduct away from output nodes OUTN and OUTP. The higher the control voltage VCTRL is, the more current passes through the transistors MSL and MSR, and the lower the transconductance of LNA 100 is. Accordingly, the power gain of the LNA 100 is variable, and can be controlled via the control voltage VCTRL.